After a string of powerful tornados struck the US Midwest and killed more than 100 people this weekend, attention has turned to the warning systems in place and why the movements of the fast-moving storms are so difficult to predict.
A tornado is a narrow, violently rotating column of air that extends from a thunderstorm to the ground, according to the National Oceanic and Atmospheric Administration. About 1,200 tornadoes hit the United States yearly.
Unlike hurricanes which can be seen gathering strength days in advance, a tornado watch lasts for four to six hours over a certain area when favorable conditions develop for tornadoes.
Hurricanes usually have diameters measured in hundreds of miles and can last for days or weeks. Scientists can usually predict a hurricane's path three to five days in advance. Tornadoes usually form in a span of a few minutes, are normally a few hundred feet wide, and last for a few minutes.